I consider a flush to be a cleaning technique. I flush after each airbrushing session. I do a a partial disassembly cleaning after about six sessions, and a full disassembly cleaning only when required.
My flush routine is as follows ( I use a bottle style brush). I take off the paint bottle (clean out later) and spray until there appears to be no paint left. Then I put on a bottle containing thinner. I usually paint with enamel, and keep a bottle with paint thinner just for cleaning. If I have just used lacquer or acrylics I pour out the paint thinner and clean bottle, then replace with appropriate thinner. I then start with a forward flush, and while doing this I place a q-tip in front of the nozzle tillit is wet, and then clean any paint off. After the spray in clear, I backflush for 25- 30 seconds and hang up brush, with the thinner bottle still on.